Timing

• Plant your dahlia tubers well after your regional frost-free date. In general, mid-April is a safe time for planting in most states. However, some areas in states such as Arizona, California, Florida, Louisiana and Nevada may have frost-free dates in February or early March, according to the Old Farmer's Almanac. Mountainous, northerly or colder regions in states such as Montana, Idaho, Michigan or Wyoming may not warm up enough until mid-May.
  
  

Temperature

• Rather than gambling by putting your dahlia tubers outside when you think spring has sprung, use a thermometer to check the soil temperature. The ideal temperature for planting dahlia tubers is 60 degrees F, according to Master Gardener Sherry Anderson at Washington State University Extension. Even if planted outside to early, your plants won't start to appear until the soil reaches this temperature. Ideally, soil will stay at between 70 and 80 degrees for a whole week for full dahlia germination.
  
  

Conditions

• Grow dahlias when your soil conditions are right. Ideal soil lets water drain easily and doesn't press tight against the roots, which can cause rotting. Heavy clay soils may stunt growth. Sand, peat moss and rotted manure or compost will improve the consistency of claylike soil. Very high nitrogen fertilizer may trigger growth in the dahlia stems and leaves at the expense of vigorous blooming, Anderson says. Low-nitrogen alternatives, such as 5-10-10, suit dahlias better than balanced versions.
  
  

Considerations

• An average frost-free date doesn't mean that a late frost can't arrive and damage your dahlias. Check forecasts before planting your tubers outside in case a cold snap is moving in. When planting dahlia tubers, choose those with a clear eye on the top nearest the dahlia crown, with a portion of neck between the crown and the tuber body. Tubers with damaged eyes or withered necks may struggle to grow.